The detection of the presence of certain vapors or liquid reactants, such as ammonia or amino compounds, is quite important, particularly when the presence of such vapors or liquid reactants indicates the presence of toxic chemicals or biological warfare agents, or the undesirable leakage of a gas from an enclosure. A number of prior apparatus and methods for detecting vapors and liquid reactants are known in the art. Nevertheless, the art does not teach or suggest a sprayable, brushable, or trowelable paint which incorporates in its dried film the combined properties of vapor permeable, liquid repellant film and an accessible indicator which can detect extremely low concentrations of reactant, such as ammonia, and regenerate the original color. More particularly, prior art does not teach the extraordinary contribution of amorphous silica as intensifier of sensitivity.